The present invention relates to a device and a method for controlling a brushless motor.
A typical brushless motor has a plurality of magnetic pole sensors, that is, magnetic pole signal output sections, each of which corresponds to one of excitation currents of multiple phases. For example, a three-phase motor includes three magnetic pole sensors respectively corresponding to U-phase, V-phase, and W-phase currents. A motor controlling device controls a brushless motor based on the timing at which rising and falling edges contained in magnetic pole signals sent from the magnetic pole sensors occur.
When there is an abnormality, for example, a breakdown, in at least one of the magnetic pole sensors, the magnetic pole signal sent from this sensor to the motor controlling device, that is, an abnormal magnetic pole signal, may include no rising edge or falling edge. In such a case, the motor controlling device may fail to properly control the brushless motor. Accordingly, different types of motor controlling devices have been recently proposed that are capable of properly controlling a brushless motor even if one of the magnetic pole sensors breaks down. For example, Japanese Laid-Open Patent Publication No. 2007-151266 describes such a motor controlling device.
When, for example, the magnetic pole sensor of the U-phase among a plurality of magnetic pole sensors breaks down, the motor controlling device of the publication temporarily stops controlling the brushless motor. Then, the rotor of the brushless motor starts coasting. During the coasting, the motor controlling device generates a simulated signal based on magnetic pole signals from the normally functioning two magnetic pole sensors (for example, the magnetic pole sensors of the V-phase and the W-phase), that is, based on normal magnetic pole signals. More specifically, the generated simulated signal includes a rising edge occurring at an intermediate timing between the rising edges contained in the two normal magnetic pole signals, and a falling edge occurring at an intermediate timing between the falling edges contained in the two normal magnetic pole signals.
If the rotation speed of the brushless motor is constant when all the magnetic pole sensors are function normally, the rising edge of the magnetic pole signal of the U-phase occurs at intermediate timing between the rising edges of the magnetic pole signals of the other two phases, and the falling edge of the magnetic pole signal of the U-phase occurs at intermediate timing between the falling edges of the magnetic pole signals of the other two phases. That is, when the rotation speed of the brushless motor is constant, a simulated signal, which is generated when the U-phase magnetic pole sensor breaks down, substantially matches with a magnetic pole signal that is output by the normally functioning U-phase magnetic pole sensor. As a result, even if one magnetic pole sensor breaks down, it is possible to properly control the brushless motor based on the simulated signal and two normal magnetic pole signals.
However, when the rotation speed of the brushless motor is not constant, that is, when the rotation of the brushless motor is accelerating or decelerating, the rising edge and the falling edge of the U-phase magnetic pole signal occur at timings different from the intermediate timing between the rising edges and intermediate timing between the falling edges of the other two phase magnetic pole signals. That is, when the rotation speed of the brushless motor is not constant, a simulated signal that is generated in the above described manner largely deviates from the original magnetic pole signal of the U-phase. Therefore, when the rotation of the brushless motor accelerates or decelerates, the brushless motor cannot be properly controlled if a simulated signal is generated.